Today's mobile communication systems are evolving rapidly. In connection with the standardization efforts in Release (Rel)) 7 for Wideband Code Division Multiple Access (WCDMA)/High Speed Packet Access (HSPA) in the third generation partnership project (3GPP), the 2-branch downlink Multiple Input Multiple Output (MIMO) was introduced for High Speed Downlink Packet Access (HSDPA). During the last years mobile operators have started to offer mobile broadband based on WCDMA/HSPA. Further, fuelled by new devices designed for data applications, the end user performance requirements are steadily increasing. Moreover, the roll out of mobile broadband has resulted in that the traffic volumes that have to be handled by the HSPA networks have grown significantly.
The growth in traffic volumes can be expected to continue. Therefore, technology that allows mobile operators to manage their spectrum resources more efficiently is of importance. One such technology that has been introduced is multi-carrier HSPA whereby carriers are pooled and handled as a common resource. Different variants of multi-carrier transmission technology have been the main features in Rel-8, Rel-9 and Rel-10. For Rel-11 new features that supplement the multi-carrier HSPA features are proposed. The new features target an increase in the (average) spectral efficiency. One such technology is multi-point transmissions for HSDPA. This is based on spatial resource pooling and it enables a User Equipment (UE) to receive data transmissions from multiple sectors and/or sites simultaneously. An aim of multi-point transmissions for HSDPA is to provide cell-edge data rate improvements.
In RAN53, four branch MIMO transmission for HSDPA was agreed as a new work item, see RP-111393, “New WI: Four Branch MIMO transmission for HSDPA”. Given a fixed amount of total downlink transmission power this would:                Double the supported peak data rate so that 84 Mbps can be supported on a single downlink 5 MHz carrier.        Improve the coverage for rank-1 and rank-2 transmissions due to the higher order of beam-forming gain.        
In Rel-7 MIMO, Primary Common Pilot Channel (P-CPICH) and Secondary CPICH (S-CPICH) are used for both coherent demodulation and Channel Quality Indicator/Precoding Matrix Indicator (CQI/PMI) estimation. When using 4 transmitting branches four common pilot signals are needed. In order to obtain a good channel estimate suitable for coherent demodulation, the pilot signal power needs to be reasonably high. To solve this, several different pilot signal design options have been discussed in 3GPP as is described in e.g. R1-122810, “Overall Summary of Pilot Design Schemes in Four branch MIMO System”:
Option1: Common Pilots Only
A solution would be to expand the current Rel. 7 solution to the 4 branch case, i.e. use common pilot signals also for coherent demodulation. To decrease the impact from the Common Pilot Channels (CPICHs) it was proposed to gate the non-legacy (3rd and 4th) CPICHs to only be transmitted when needed for data demodulation and periodically to allow for CQI/PMI estimation, see R1-121755, “Four branch MIMO Performance with Common Pilot Gating”.
Option2: Common Pilots+Dedicated Pilots
In accordance with this option, there are still four common pilot channels but only used for CQI/PMI estimation. Besides the common pilot channels, dedicated or UE specific pilot channels are introduced for High-Speed Downlink Shared Channel (HS-DSCH) demodulation. The dedicated pilot signals are UE-specific and only need to be received by that specific user in a certain area. In this way, dedicated pilot signals can be set to have a relatively low transmit power if that is enough for that specific user. The power of the non legacy (3rd and 4th) common pilots can be reduced as CQI/PMI estimation requires less channel estimation accuracy.
Option3: Common Pilots+Scheduled Pilots
In accordance with this option, the four common pilot channels are still only used for CQI/PMI estimation. In addition, two additional 3rd&4th pilot signals are transmitted when there is 4×4 data transmission on HS-DSCH to assist data de-modulation together with the existing common pilot signals. The additional pilot signals are not UE-specific and not precoded. The power of the non legacy (3rd and 4th) common pilots can be reduced as CQI/PMI estimation requires less channel estimation accuracy.
For option 1, pilot gating was not accepted in 3GPP as it also leads to higher interference variations especially to legacy UEs, thus a decreased performance. Still similar problems will exist with co-existence of 4 branch and legacy UEs.
There is a constant demand for improving existing systems and to provide more efficient transmission in a cellular radio system. Hence, there is a need for a method and an apparatus that provide an improved transmission in a cellular radio system.